Metal phosphate coating solutions are dilute solutions of phosphoric acid and other chemicals which are applied to the surface of metal; the surface of the metal reacts with the solution and forms an integral layer (on the surface of the metal) of substantially insoluble crystalline phosphate. This layer is applied primarily for protection from corrosion, or as a base for the application of a second coating (e.g., paint), or as a vehicle to retain a lubricant on the coated surface.
Metal phosphate coatings are well known as being useful in the forming of metals. The metal phosphate coating, when applied to the surface of an article about to be subjected to formation, reduces the friction created by drawing or cold forming operations; the coating reduces the great amount of friction between the metal surface and the die. The conversion of the metal surface to a phosphate coating reduces this friction primarily by increasing the ability of the metal to retain a uniform film of lubricant over the entire surface; this ability to retain a lubricant is critical--it is this second lubricant which actually prevents welding and scratching in drawing operations, and reduces metal to metal contact in cold forming operations. This reduction in friction allows shapes to be made by cold forming which would otherwise not be possible or practicable.
The present invention relates to a cold deformation process which employs phosphate coating solutions particularly suited to applying a lubricating (or lubricant-retaining) phosphate coating. The unique character of this coating stems from its application or deposition from a coating solution employing hydroxylamine.
The use of hydroxylamine in coating solutions is known in the art.
U.S. Pat. No. 2,298,280, issued Oct. 13, 1942, discloses the use of hydroxylamine in phosphate coating solutions as an accelerator to assist in depositing corrosion-resisting paint base phosphate coatings. However, the absence of nitrate, in combination with relatively low levels of zinc and phosphate, render the exemplified solutions capable of providing lubricating coatings which are only marginally effective for undemanding cold forming processes (e.g., wire and tube pulling); such solutions would be wholly unacceptable for providing lubricating phosphate coating for metal surfaces about to undergo demanding cold deformation operations (e.g., the extrusion of large parts).
U.S. Pat. No. 2,702,768, issued Feb. 22, 1955, describes the use of hydroxylamine in "noncoating phosphate" solutions, such as those which contain sodium, potassium and ammonium phosphates.
U.S. Pat. No. 2,928,762, issued Mar. 15, 1960, discloses the use of hydroxylamine phosphate as a reducing agent in an orthophosphoric acid preliminary rinse solution in a phosphate coating process.
U.S. Pat. No. 4,003,761, issued Jan. 18, 1977, discloses a process for applying a phosphate coating to a ferric surface. This process comprises spraying the surface with an aqueous acid solution having a pH of 4.3 to 6.5 and which contains an alkali metal or ammonium salt. The solution additionally contains an oxidizing or reducing agent accelerator, from 0.05 to 1.0 gram per liter of a C.sub.2 -C.sub.4 alkylolamine, and a wetting agent.
U.S. Pat. No. 4,149,909, issued Apr. 17, 1979, discloses accelerated phosphatizing compositions containing hydroxylamine sulfate (as a source of hydroxylamine in the use solution) to provide a hydroxylamine plus chlorate/bromate accelerator combination.
U.S. Pat. No. 4,220,486, issued Sep. 2, 1980, discloses conversion coating solutions having a pH of 5.5 to 6.5 which optionally employ 0.2 to 5.0 grams per liter of pyrazole, hydroxylamine or hydrazine compounds. These compounds are added to stabilize the use solutions.
Thus, the art has recognized that hydroxylamine, or hydroxylamine salts or complexes, can be employed to assist in depositing coatings designed to serve as a paint base, or as a corrosion-inhibiting base. However, it has now been surprisingly discovered that moderate to heavy phosphate coatings applied from a phosphate coating solution which contains hydroxylamine are particularly well suited to act as lubricating (or lubricant-retaining) coatings on ferrous-base metal surfaces which are about to undergo cold deformation. It has also been surprisingly discovered that phosphate coatings particularly suited for prelubricantion of articles about to undergo cold deformation can be deposited even in the presence of ferrous, or ferrous and ferric, ions, when hydroxylamine is employed.